The present invention relates, in general, to photonic devices, and more particularly to improved multi-level integrated photonic devices and methods for fabricating them.
In the past, semiconductor lasers were typically fabricated by growing the appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an active layer parallel to the substrate surface. The material was then processed with a variety of semiconductor processing tools to produce a laser cavity incorporating the active layer, and metallic contacts were then attached to the semiconductor material. Finally, laser mirror facets were formed at the ends of the laser cavity by cleaving the semiconductor material to define edges or ends of a laser optical cavity so that when a bias voltage was applied across the contacts, the resulting current flow through the active layer would cause photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow.
An improvement over the foregoing process was described in U.S. Pat. No. 4,851,368, which discloses a process for forming mirror facets for semiconductor lasers by a masking and etching process that allowed lasers to be monolithically integrated with other photonic devices on the same substrate. This patent also teaches that total-internal-reflection facets can be created within an optical cavity through the fabrication of such facets at angles greater than the critical angle for light propagating within the cavity. The ability to fabricate multiple photonic devices on a single substrate led to the fabrication of complex integral optical circuits in which multiple active and passive optical devices are integrally fabricated on a single substrate. Such optical circuits may incorporate integrated lasers, waveguides, detectors, semiconductor optical amplifiers (SOA), gratings, and other optical devices.
Recently, there has been tremendous interest in developing an electroabsorption-modulated laser (EML) through the integration of a laser and an electroabsorption modulator (EAM). However, existing methods of fabricating monolithic EML devices typically have involved semiconductor regrowth steps to separately fabricate the laser and the EAM, but such methods have resulted in poor yields and high costs.
Copending U.S. patent application Ser. No. 10/226,076, filed Aug. 23, 2002, entitled “Wavelength Selectable Device” and assigned to the assignee hereof, discloses a method of incorporating monolithic structures such as an electroabsorption modulator coupled with a laser cavity on a substrate without the need for epitaxial regrowth.
Another example of an integrated EML device is described in U.S. Pat. No. 6,483,863, wherein the EML comprises two stacked asymmetric waveguides, the first waveguide forming a laser and the second waveguide forming an EAM. The two waveguides support two different modes of light propagation and are arranged so that light propagating in the first waveguide is transferred into the second waveguide via a lateral taper in the first waveguide. However, due to the use of a lateral taper to transfer light propagating in the laser to the EAM waveguide, close proximity of these two waveguides is required, resulting in a reduced confinement factor for each quantum well in the laser.
A very important factor in determining laser performance is its confinement factor Γ for each quantum well in the laser. A smaller value of Γ leads to higher threshold currents for lasing and results in a higher amount of dissipated heat by the laser. Reducing heat dissipation by lasers is a key requirement of modern-day lasers and is very important to a viable EML product. A modal analysis for a typical laser structure including a metal contact layer on the top, or p-side of the laser indicates a confinement factor Γ of 2.55% for each quantum well in the laser. A modal analysis of a structure similar to that of U.S. Pat. No. 6,483,863, including both the laser and underlying EAM, but also including the p-side metals, results in a confinement factor of 1.37% due to the proximity of the EAM, which is required since the electroabsorption modulated laser (EML) is formed by transferring light propagating in the laser waveguide to the EAM waveguide via a lateral taper in the laser waveguide. The result is a laser device having suboptimal performance.